The present invention relates to all-optical signal regeneration and reshaping techniques, and more particularly, to a photonic integrated subcircuit having a honeycomb architecture for performing photonic signal regeneration and re-shaping (P2R) in the counter-propagative or co-propagative operation. The present invention further relates to an integrated optical circuit comprising multiple such subcircuits for performing multi-channel P2R.
P2R is an important function that lends itself to photonic integration. Photonic integrated circuits (PICs) realize several functions such as amplification, splitting, combining, filtering, and grooming on a single chip, and are key enablers of cheap and efficient network operation. All-optical regeneration and reshaping overcomes many limitations of electrical or optoelectronic counterparts, such as limitations on data rate, cost, flexibility, footprint, power consumption, etc. FIGS. 1a and 1b illustrate using a Mach-Zehnder (MZ) interferometer to perform P2R in counter-propagative and co-propagative operation modes, respectively.
FIG. 1a shows an co-propagative operation mode in which an input signal S from an arm 33 entering an arm 32 of a MZ interferometer at a coupler 23 is regenerated as an output at a coupler 21 by interference at coupler 21 between a continuous wave (CW) signal from arm 34 traveling through two optical paths or arms 31 and 32 of the interferometer. Each arm 31, 32 is coupled to a semiconductor optical amplifier (SOA) 12, 13 respectively, which works as a phase shifter as well as a signal amplifier. The input signal S interacts with the CW signal in arm 32 and causes it to change phase. Due to the well-known Cross-phase modulation (XPM) effect, the CW signal in arm 32 has a phase difference when the input signal S is in its high than when the input signal S is in its low. This phase difference induced by XPM is tuned to be 180 degrees, e.g., by tuning drive currents in the SOAs. The phase of the CW signal from arm 31 remains the same at the coupler 21, irrespective of whether the input signal S is in its high or low state.
In addition, the drive currents of SOAs 12, 13 in the arms 31, 32 are tuned such that, when the input signal is in its low, the CW signals from the two arms 31, 32 are 180 degrees out of phase at the coupler 21. Thus, when the input signal is low, the CW signals from the two arms 31, 32 destructively interfere with each other and generate a “0” output at the coupler 21, and when the input signal is in its high, the phase of the CW signal in the arm 32 is flipped by the input signal S, thus constructively interferes at the coupler 21 with the CW signal from the arm 31 and generates an high output. If the amplitude of the CW signals out of arms 31, 32 is “A”, the amplitude of the overall output generated at the coupler 21 will be 4A2. Thus, the input signal S is regenerated as the overall output at the coupler 21 by interference between the CW signals from the arms 31, 32.
However, because the input signal S, which is modulated in the CW signal in arm 32, also reaches the output arm 35 through the arm 32, it represents a source of noise and would need to be filtered out. This would not be possible if the input signal S and the CW signal input are at the same wavelength. Therefore, a wavelength conversion has to be performed in co-propagation based devices.
FIG. 1b shows a circuit operates in a counter-propagative mode. As illustrated in FIG. 1b, a CW signal from arm 35 enters an MZ interferometer at the coupler 21, thus the CW signal in the arm 32 travels in a direction opposite to that of the input signal S traveling in arm 32. Similar to the co-propagative mode, the drive currents of SOAs 12, 13 in the arms 31, 32 are tuned such that, when the input signal is in its low, the CW signals from the two arms 31, 32 have 180 degree out of phase at the coupler 22. Thus, when the input signal S is in its low, the CW signals from the two arms 31, 32 destructively interfere with each other and generates a “0” output at the coupler 22, and when the input signal is in its high, the phase of the CW signal in the arm 32 is flipped by the input signal S, thus constructively interferes at the coupler 22 with the CW signal from the arm 31 and generates an high output. Therefore, the input signal S is regenerated as the overall output at the coupler 22 by interference between the CW signals from the arms 31, 32.
For the device to function effectively as an amplifier, it needs to support weak input signals. However, to flip the phase of the CW signal in the arm 32 when the input signal S is in its high, the input signal must be strong enough, i.e., approximately equal in intensity to the CW signal in arm 32. To this end, a preamplifier SOA 15 is provided to preamplify the input signal S before it enters the arm 31 at the coupler 23. Therefore, the SOA 15 in arm 33 needs to be effective in amplifying a weak input signal S. However, the CW signal in the arm 32 also reaches SOA 15 via arm 33 through the coupler 23. Since the CW signal in arm 32 is typically much stronger than the input signal S, it saturates the SOA 15. Therefore, SOA15 cannot pre-amplify the weak input signal.
Therefore, there is a need for an improved photonic integrated circuit that can overcome the above shortcomings in the prior art.